Paint-spraying is carried out on a large scale in the automobile industry or in all other industries producing apparatus and equipment, where sheet metal members have to be painted in large quantities, as for domestic appliances, electrical appliances, machine panelling or the like. In this technique, the coating medium is atomized, for instance with the aid of a spray gun, and the spray of very fine droplets is directed onto the article to be painted. Since a high quality finish requires that the article be coated evenly, also in the edge areas, part of the spray of droplets goes past the article to be painted and strikes a wall of the spray booth. The fine spray or mist of paint droplets in the air of the spray booth not only reaches the walls of the spray booth itself, but also gets into the drain pipes and the following units. It is therefore necessary to largely prevent or remove these deposits of paint droplets on the booth walls, in the drain pipe connections and the following units. To this end, the respective walls or the like are generally supplied with water, so that a closed film of water runs down the walls, taking the droplets with it as they strike the wall. The paint-laden fluid collecting in the lower zone of the spray booth then passes into a separating vessel to separate the paint from the aqueous phase and to recycle this aqueous phase to the spray booth.
If water without any additives is used for extracting the paint droplets, then this leads to particles of paint reaching the spray booth walls through the water film and adhering thereto or to the drain pipe connections and the following units; it may also lead to plugging of the nozzles or the like with whose aid the aqueous phase recycled to the spray booth is distributed over the walls.
To solve this problem, one has alrady used salts of sulfonic acids (U.S. Pat. No. 2,208,657), reaction products of starch with formaldehyde (German Pat. No. 836 759), polyvinyl compounds and derivatives of high-polymer carbohydrates (German Auslegeschrift No. 10 17 957), polyamines, polyacrylamides, polyacrylic acids, polymethacrylic acids and polyoxyethylenes (French Pat. No. 15 13 413), oxalic acid (German Offenlegungsschrift No. 22 00 807)or also salts of carboxylic acid (German Offenlegungsschrift No. 23 47 068). In such spray units, droplets of paints capable of being thinned with water were extracted from the fine spray or mist using as aqueous phase solutions of salts having a monovalent, bivalent or trivalent cation. Calcium, barium, manganese, zinc, copper, lead, cadmium and cobalt combined with the anions sulfate, chloride, acetate, tartrate, citrate, phosphate or nitrate were used as bivalent cations. In the prior art process, the salts of bivalent metals had a concentration of 0.4 to 2% by weight in such wash solutions, attention having to be paid that the pH value of this wash solution was not less than 2 and not more than 10. With the aid of this wash solution, the paint droplets coagulated. The coagulate could be separated from the aqueous phase, i.e. from the wash solution. Special control of the pH value is not necessary in this form of prior art.
In the case of all these prior art measures for paints incapable of being thinned with water, the paint coagulate collects at the bottom of the separating vessel. German Auslegeschrift No. 24 33 193 has disclosed that for paints capable of being thinned with water, a concentrated calcium chloride solution of about 40% by weight and having a density of about 1.43 g/cm.sup.3 be used as washing fluid. Owing to the high density of the aqueous phase, the paint coagulate floats to the top of the separating vessel and can be skimmed off the aqueous phase. This prior art process has two major drawbacks, namely, the costs of the large amounts of calcium chloride required are considerable and, in addition, a highly concentrated calcium chloride solution is a corrosive fluid which acts on the separating vessel, the relative piping, the booth walls and the like. It is not possible for this prior art process to be used for paints incapable of being thinned with water.
Finally, liquid coagulating agents or flocculating agents are on the market which have to be strongly alkaline, so that such wash solutions have to be expected to have a pH value of at least 11 in order for the paint droplets to be coagulated. These liquid coagulating agents are used both for paints containing metal pigments (metal effect or metallic paints) and for paints containing colour pigments ("Uni" paints). The drawback with these prior art coagulating agents, particularly in the case of metallic paints, is the high pH value which leads to the metal pigments, e.g. aluminum bronze, decomposing or dissolving and generating hydrogen. For this reason the precipitated paint coagulate is frothy and discharge thereof by the conventional discharging means, e.g. scraper bands, cannot be effected or can only be effected in an unsatisfactory manner. Apart from this, the generation of hydrogen represents an explosion hazard which is not to be underestimated, particularly in view of solvent vapour possibly being present. The high pH value of this prior art liquid coagulating agent also leads to the paint itself undergoing change or decomposing, the metal pigment being separated and deposited and the paint base of the coagulate becoming less viscous, this impeding flotation of the coagulate. In this case the paint coagulate floats up due to the wash solution, which contains droplets of paint and runs down the walls of the spray booths of the like, entraining air which attaches to the paint droplets in the form of small bubbles and in this way causes the paint coagulate to float to the top of the thus regenerated wash solution. A further drawback encountered with this prior art process using strongly alkaline liquid coagulating agent is that this wash solution cannot be drained off into the municipal waste water system, but first has to be neutralized to a pH value of at most 9.5 in line with statutory regulations. This requires considerable amounts of acid and the provisions needed for using and handling these amounts of acid represent a handsome cost factor.